While the term ADD, or attention-deficit disorder, has become increasingly popular among laypersons,
ADHD, or attention-deficit/hyperactivity disorder, is the only clinically diagnosed term for disorders characterized by inattention, hyperactivity, and impulsivity used in the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorder, Fourth Edition.
There are actually three different types of ADHD, each with different symptoms: predominantly inattentive, predominantly hyperactive/impulsive, and combined.

Some professionals use the term ADD to refer to the predominantly inattentive type of ADHD.

A legitimate mental disability often viewed as a made up excuse for poor performance in school (which it frequently is.) It is natural for children to display signs of ADD, this does not however mean they are afflicted. It means they are normal, well-balanced human beings. Symptoms generally fade in the early teens; if by 15 an individual's condition persists or worsens, they may (but not always) be justly diagnosed. However, for those who genuinely suffer, the clarity of mind the medication can provide is priceless and worth irritating fakers who discredit the illness.
This is not a sign of stupidity; in fact, ADD frequently affects those with above average IQ's, such as Ivy League students, and many renowned geniuses throughout history. Though the occasional genuine moron and/or average thinker will fall victim.
A test of whether or not the condition is actual could be if the individual in question has difficulty focusing on the activities they enjoy as well as activities they dislike.

While his fellow students have learned to concentrate on assignments, Alex remains unable to focus on the simplest task.

Contrary to popular belief, it is not just a lame excuse for misbehaving children. While it is sometimes labeled as so by people who don't know what they are talking about, it is an actual disorder that has more effects besides misbehavior, including difficulty concentrating and restlessness, and should not be discounted amongst cynics with nothing better to do than whine on the internet.

I am a 16-year-old with ADHD that really does have difficulty concentrating who takes medication that does actually help, and is not called ritilin.

A MMORPG word used to tell other players that a player has picked up a few bad guys and they are following him/her to their location. It is a way of telling others that your bringing in the nasties. You are coming in hot.

Player 1: "I coming in with adds!"
Player 2: "How many?"
Player 1: "The whole HORDE!"